The present invention relates to a process for operating an electro-optical glare shielding device and a glare shielding device suitable for implementing this process.
Glare shielding devices are universally known and are used preferably in the welding and cutting torch technology. As a rule, with these glare shielding devices the radiation above 780 nm (infrared) and below 365 nm (ultraviolet) are filtered and only the radiation in the visible range is dimmed.
Safety shields are known, for example, from French Patent No. FR-2 293 188, where, in particular, safety shields are described that are constructed of an ultraviolet light filter, an infrared filter, a polarizer and an analyzer, between which is an electro-optical element. The electro-optical element rotates the polarizing direction of the light polarized by the polarizer into a direction for which the analyzer is opaque. Thus, within a few tenths of a second adequate dimness can be obtained. During this dimming period, however, the user is intensively blinded and is thus inadequately protected.
Therefore, electronic circuits, as described for example in U.S. Pat. No. 3,575,491, have already been developed to operate liquid crystal cells, with which the change-over times of the liquid crystal cells are in the millisecond range. To this end, a high electric voltage, which fluctuates with a frequency of over 60 Hertz, is attached to the liquid crystal cell. Unfortunately the operation of these devices is associated with a high electric power and thus the properties of the liquid crystal cells that are used are rapidly modified.
These and all other prior art glare shielding devices are characterized by a high power consumption. Additionally, the drawbacks of devices with unstable or rapidly consumed voltage sources are well known.
Since today's conventional glare shielding devices utilize a plurality of liquid crystal cells, usually several connected in series, in the steepest range of the transmission characteristic of these liquid crystal cells, the high temperature dependence and the high voltage dependence of this characteristic have a high negative effect. In particular, during normal operation these dependencies render the use of automatically starting glare shielding devices difficult and make it necessary to compensate for these changes in transmission.
Another problem that has still remained unsolved relates to the scattered light which is produced by the liquid crystal cells themselves and cannot be eliminated by the polarizers.